Global Positioning System (GPS) provides a worldwide, 24 hour, location service. The system includes multiple GPS satellites to broadcast location signals, control stations to monitor and control the satellites, and a GPS receiver to receive the signals. Commercial Global Positioning System (GPS) receivers now are used to provide accurate location information in many navigation, tracking, and timing applications. A GPS antenna that is a part of a GPS receiver must have a line of sight to a GPS satellite to receive the GPS signal from that satellite.
GPS location is based on one-way ranging from the GPS satellites to the GPS antenna. Ranges are measured to four satellites simultaneously in view by tracking (correlating) the frequency and the time of arrival (TOA) of the incoming GPS signal to a receiver-generated replica signal. With four ranges, the receiver can determine four unknowns, typically latitude, longitude, altitude, and an adjustment to the replica. The ranges are called "pseudoranges" because the actual distances to the GPS satellite are not known until the internal replica has been adjusted. Time of day is computed from the adjustment to the TOA of the replica. If the receiver is given other information, such as an altitude, or a precise time, fewer than four pseudoranges are needed.
Each GPS satellite broadcasts its position in a signal having a carrier frequency at approximately 1.575 GHz. The signal is modulated by a PRN sequence of 1023 chips, repeating at a 1 millisecond time interval. Each satellite uses a different PRN sequence, thus enabling a GPS receiver to distinguish the GPS signals from the different GPS satellites. The frequency of the signal received from each GPS satellite will have a Doppler shift due to the relative velocity between the GPS satellite and the GPS antenna. A velocity and a direction for the GPS antenna may be determined from the rate of change of the location or from the rate of change of the pseudoranges after accounting for the Doppler shift due to the motion of the satellite.
Power consumption is an important figure of merit for a GPS receiver. A low power consumption is good for a GPS receiver that depends upon a battery for a power source. To achieve low power, some GPS receivers have a normal mode where the GPS signal is correlated and a location fix is provided and a standby mode where power consumption is lower than in the normal mode. Some GPS receivers reduce the average power consumption in the receiver by alternating between a time duration in the normal mode and a time duration in the standby mode. A limitation of the GPS receivers using a standby mode to reduce power consumption is that the GPS receiver cannot correlate the GPS signals or provide a location fix during the time duration in the standby mode.
What is needed is a GPS receiver apparatus and a method for reducing power in a GPS receiver while the GPS receiver continues to correlate GPS satellite signals and to provide a location fix to a user.